7A.4 Uncertainty in modeling dust mass balance and radiative forcing from size parameterization

Zhao, Chun, Siyu Chen, L. Ruby Leung, Yun Qian, Rahul Zaveri, Pacific Northwest National Laboratory, Jasper Kok, University of California Los Angeles, and Jianping Huang, Lanzhou University, China

This study examines the uncertainties in simulating mass balance and radiative forcing of mineral dust due to biases in the aerosol size parameterizations. For this purpose, the simulations are conducted semi-globally (180oW-180oE and 60oS-70oN) using the WRF-Chem model with three different aerosol size approaches (8-bin, 4-bin, and 3-mode). The biases in the 3-mode or 4-bin approaches against a more accurate 8-bin approach in simulating dust mass balance and radiative forcing are identified. Compared to the 8-bin approach, the 4-bin approach simulates similar but coarser size distributions of dust particles in the atmosphere, while the 3-mode approach retains more fine dust particles but fewer coarse dust particles. Although the 3-mode approach yields up to 10 days longer dust mass lifetime over the remote oceanic regions than the 8-bin approach, the three size approaches produce similar dust mass lifetime (3.2 days to 3.5 days) on semi-global average, reflecting that the global dust mass lifetime is mainly determined by the dust mass lifetime nearby the dust source regions. With the same global dust emission (~6000 Tg yr-1), the 8-bin approach produces a dust mass loading of 39 Tg, while the 4-bin and 3-mode approaches produce 3% (40.2 Tg) and 25% (49.1 Tg) higher dust mass loading, respectively. The difference in dust mass loading between the 8-bin approach and the 4-bin or 3-mode approaches has a large spatial variability, with generally smaller relative difference (<10%) near the surface over the dust source regions. The three size approaches also result in significantly different dry and wet deposition fluxes and number concentrations of dust. The difference in dust aerosol optical depth (AOD) (a factor of 3) among the three size approaches is much larger than their difference (25%) in dust mass loading. Compared to the 8-bin approach, the 4-bin approach yields stronger dust absorptivity, while the 3-mode approach yields weaker dust absorptivity. Overall, on semi-global average, the three size parameterizations result in a significant difference of a factor of 2~3 in dust surface cooling (-1.02~-2.87 W m-2) and atmospheric warming (0.39~0.96 W m-2) and in a tremendous difference of a factor of ~10 in dust TOA cooling (-0.24~-2.20 W m-2). The uncertainty in dust emission estimation due to the size parameterizations is also quantified as a factor of 2. This study highlights the uncertainties in modeling dust mass and number loading, deposition fluxes, and radiative forcing resulting from different size parameterizations, and motivates further investigation of the impact of size parameterizations on modeling dust impacts on air quality, climate, and oceanic ecosystem.